The goal of this study is to utilize recent developments in the molecular biology of circadian rhythms to better understand circadian functioning and circadian disorders in humans. The investigators will measure the endogenous circadian period (tau, the length of the biological day) in a unique population of 50 blind subjects that allows for very precise phenotyping. They will also tau phenotype 260 sighted subjects by measurement circadian phase and phase angle of entrainment. After tau phenotypes have been defined in each population, they will determine whether similar clock gene alleles predict phenotypes in the two populations. It is hypothesized that similar alleles predict diurnal preference and phase angle of entrainment in sighted subjects and circadian period in blind subjects. The candidate, Jonathan S. Emens, M.D., plans to become an independent investigator in the area of circadian physiology and genetics. He wishes to better understand the physiologic and genetic basis of diurnal preference in sighted humans and 24-hour sleep/wake schedule disorders in the blind. He plans to continue research into the basic mechanisms of the circadian system, illnesses with a circadian etiology and diseases impacted by circadian functioning. He plans to pursue a career in sleep medicine. Dr. Emens will be sponsored by Alfred J. Lewy, M.D., Ph.D., and Susan Hayflick, M.D., for this career development award. Under Dr. Lewy s guidance, Dr. Emens will expand his existing knowledge base in the area of circadian physiology and will receive training in techniques of human circadian research and the clinical impact of the circadian system. Dr. Hayflick will help Dr. Emens to develop an understanding of the theory and techniques of molecular genetics. In addition, Dr. Emens will complete a rigorous didactic program in molecular genetics and clinical research techniques. Coursework will be completed in biochemistry, molecular genetics, eukaryotic cell biology, molecular cell biology techniques, biostatistics, legal and ethical aspects of clinical research, scientific writing, randomized trial research and medical informatics.